How to turn off Gmail's spam filtering

If you are wondering why you may need this, it might be because you just want all your mail in a single folder and don't want any protection against spam or maybe because you are forwarding to another ID (and in that case spam will not be forwarded); any mail that mistakenly goes to the forwarding ID's spam folder stays there. It is better to consolidate all your mail in the final inbox and use the spam protection of that service. Same probably goes for when you are downloading your mail using your email client; you will want your email client or receiving ID's mail service to handle the spam instead of gmail.

There's a simple way of not letting any of your incoming mail to go to spam folder. This is the way I have been using for my forwarding IDs so that even the spam mail is forwarded and the receiving ID decides what is spam and what is not:

Go to "settings". Create a filter, in the "Doesn't have" section type a long random string, eg: "klhfsdujfhrelkfhu4eihvkfdlhkjghdflhgfh".

Click "Create filter with this search", check "never send it to spam". The resulting filter will look like:

Matches: -klhfsdujfhrelkfhu4eihvkfdlhkjghdflhgfh
Do this: Never send it to Spam

This tells gmail never to send any email to spam unless it has the given string (there's lesser chance that one of your incoming emails will contain the same exact string than you getting hit by lightening without a thunderstorm).

Another way (that I never tried but obviously seems correct) is to type "is:spam" in the "has the words" section and click never send it to spam so that it would look like:

Matches: is:spam
Do this: Never send it to Spam